This toy keyboard is being used to play music on an NES. As you probably already know, the hardware inside those original controllers was dead simple. They’re just a parallel to serial shift register that reads from all of the keys. To get this keyboard up and running [heavyw8bit] simply mounted eight chips inside the gutted toy, connecting two of them to the keyboard keys, and the rest to the array of push buttons he added to the right.
So what’s the point of using this as a quadruple game controller? Are you expecting to see what a full speed-run of Contra looks like using this as the controls? That’s not the point at all. This becomes a musician-friendly frontend for the NES synthesizer ROM called NESK-1. [heavyw8bit] wrote the game/program in order to allow you to use the original console hardware to play all of the sounds you know and love. Our favorite is the arpeggio example heard at about 2:35 into the clip after the break.